Rogue Planets
There's a fair amount of talk going on right now about rogue planets wandering about between the stars. None of this surprises me in the least, because when I was high school physics, we had to mess around with a solar system building program. It was THE BANE OF MY HIGH SCHOOL CAREER. Even the physics nerds were getting frustrated with it. Every single time you set something up, all the planets would go rocketing out into space.